How can I use a method to return the index of an array element?

Question

I am trying to use a method to return the value of the index of an array so I can use it in another class, but I can't seem to make it work. Here is my code:

This one tells me there is no return statement.

public int getCourseIndex(String course){
    for (int i = 0; i<courses.length; i++)
       if(course.equals(courses[i])) 
}

I also tried and I think it just returns 0:

public int getCourseIndex(String course){
    int total = 0;
    for (int i = 0; i<courses.length; i++){
        if(course.equals(courses[i])){
            total++;
    }

    return total;
}

The easiest way would be to create an int index variable. In your for loop, if you found a course, set this value to i and break the loop using a break; — Alexis C.
– Alexis C., Commented Dec 10, 2013 at 8:05
@user2848565 You can read it here : docs.oracle.com/javase/tutorial/java/nutsandbolts/branch.html. Additionally, like answers have mentionned, if you only need to return the index, a return statement is sufficient. — Alexis C.
– Alexis C., Commented Dec 10, 2013 at 8:08
@user2848565 Don't forget that with your approach & the others suggested you only get the index of the first occurrence of the String in your array. If that String occurs multiple times and you want to know all those indices, you may want to return an ArrayList. — async
– async, Commented Dec 10, 2013 at 8:17

zbr · Accepted Answer · 2013-12-10 08:22:39Z

3

You need to go through the array with a for loop and if you find what you were looking for, return the current loop variable value (in the following code the i value), which actually represents the index in the array on which it was found.

If the loop ends and nothing gets returned, it means that what you were looking for wasn't in the array. Then you should return something to inform you about that fact. It needs to be something that can't get returned from inside the for loop, which are negative numbers (typically a -1).

public int getCourseIndex(String course){
    for (int i = 0; i<courses.length; i++){
        if(course.equals(courses[i])){
            return i;
        }
    }
    return -1; // not found
}

edited Dec 10, 2013 at 8:22

answered Dec 10, 2013 at 8:08

zbr

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2 Comments

async Over a year ago

There's no closing brace for the for loop:D

zbr Over a year ago

@user16547: Oh, thank you. But to defend myself, he didn't have it in the question either. ;)

Infinite Recursion · Accepted Answer · 2013-12-10 08:12:03Z

2

You can use java.util.Arrays as below:

public int getCourseIndex(String course) {
    return (Arrays.asList(courses)).indexOf(course);
}

Or, if you want to calculate using loop, you can:

public int getCourseIndex(String course) {
    for (int i = 0; i < courses.length; i++)
        if (course.equals(courses[i])) {
            return i;
        }
    return -1;
}

answered Dec 10, 2013 at 8:12

Infinite Recursion

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1 Comment

chronodekar Over a year ago

I like this answer. Exposes the OP to Arrays - a useful look-ahead.

Alexis C. · Accepted Answer · 2013-12-10 08:18:58Z

1

Additionally, you can use ArrayUtils.#indexOf(Object[] array,Object objectToFind) from org.apache.commons.lang.ArrayUtils.

Finds the index of the given object in the array. This method returns INDEX_NOT_FOUND (-1) for a null input array.

public int getCourseIndex(String course){
  return ArrayUtils.indexOf(courses, course)
}

answered Dec 10, 2013 at 8:18

Alexis C.

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Comments

Damian Leszczyński - Vash · Accepted Answer · 2013-12-10 08:08:55Z

0

To return the index you should traverse the loop and return the i when your validation results true.

If not found you have two option, return -1 or throw an exception.

public int getCourseIndex(String course){

 for (int i = 0; i < courses.length; i++){
   if(course.equals(courses[i])){
    return i;
   }
 }
 return -1;
}

answered Dec 10, 2013 at 8:08

Damian Leszczyński - Vash

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